The undercover investigation was conducted last month and the groups released their final report Thursday.

“Apple and Catcher have done little to ensure that safety standards and other conditions have improved for Catcher’s workers,” the report said. It wants Apple to live up to its supplier code of conduct, which sets rules for the treatment of workers.

The investigation found 22 labor violations, CLW said, including fire exits locked shut, workers not given equipment to protect them from toxic chemicals, and mandatory overtime.

“Workers are made to labor for up to 100 hours of overtime per month, nearly three times the limit of 36 hours mandated by Chinese Labor Law,” the report says. Workers in some cases were not paid for overtime work.

Apple said it was committed to taking quick action. “We know our work is never done, and we are devoted to constant improvement,” it said.

The Suqian facility makes aluminum enclosures for MacBooks and iPads, Apple said. It said it conducts quarterly fire-safety inspections at the plant, and that the factory repaired fire extinguishers and unblocked fire exits as recently as last week.

“Our most recent annual audit, in May, found some concrete areas for improvement in Catcher’s operations, and we worked with Catcher to develop a corrective action plan,” Apple said via email. “We had scheduled a follow-up visit next month to review their progress but have dispatched a team there immediately to investigate this report.”

The Catcher factory also makes products for Dell, Hewlett-Packard, Lenovo, HTC and Motorola, CLW said. The parent company, Catcher Technology, is based in Taiwan and operates multiple factories in China.

In its most recent supplier responsibility report in February, Apple said more factories were complying with the 60-hour work week rule. The company conducted 451 supplier audits in 2013, it said.

“Excessive overtime is not in anyone’s best interest, and we work closely with our suppliers to prevent it. We track and report the weekly working hours for more than 1 million workers, and, through the end of August, Catcher has averaged 95 percent compliance with our 60-hour work week limit this year,” Apple spokesman Chris Gaither said.

Other issues highlighted in the CLW investigation included workers being “required to pay for drinking water and showers in dorms,” and a lack of representation for expressing grievances to upper management.